1. Field of the Invention
The present invention relates to a disc recording and/or reproducing apparatus and more particularly to a disc recording and/or reproducing apparatus in which a recording and/or reproducing head is moved radially with respect to a disc-like recording medium to record or reproduce information into or from the recording medium.
2. Description of the Prior Art
For example, a magnetic disc recording and/or reproducing apparatus is such an apparatus that has a magnetic head to be moved radially relative to a magnetic disc which is rotated so that information is magnetically recorded or reproduced along a track defined on the magnetic disc.
In such an apparatus of the type described above, the accuracy of the displacement of a magnetic head; that is, the accuracy of positioning a magnetic head at a desired position on the disc is dependent upon the accuracy of a pulse motor as a driver for shifting the magnetic head and the machining tolerances of various parts of the apparatus. In this regard, the accuracies of such pulse motor and various parts are strictly controlled.
Even if high precision parts are used, there arises a problem of accuracy of assembling these parts. If assembly tolerances vary,, it is of no use to use high precision parts. In addition, a high assembling accuracy leads to an increase of manufacturing cost.
As a mechanism for displacing a magnetic head with respect to a magnetic disc, there is known a mechanism in which a head mount having a magnetic head mounted thereon is displaced through a cam by a pulse motor. However, unless the guide mechanism of the head mount is compact in size and light in weight, a large force is exerted on the cams and the pulse motor. As a result, the output of the pulse motor must be increased and the mechanical strength of the cam mechanism must be strengthened. This means that the pulse motor and the cam mechanism become complicated in construction, large in size and heavy in weight, resulting in a high manufacturing cost.
In addition, the magnetic head must be accurately positioned at a desired track. Especially, the same magnetic disc is used by various magnetic disc recording and/or reproducing apparatuses and "compatibility among magnetic discs is required. Therefore magnetic head must be accurately mounted on the head mount.
From this viewpoint, it is desired that the tolerance of the head mount with which the magnetic head is mounted on the head mount as well as the deviation of the position of a head chip relative to the head mount must be correctly adjusted.
Furthermore, there is a positioning mechanism of the magnetic head in which a cam is arranged around a rotating axis of the magnetic disc so that the cam serves to position the magnetic head at a desired track on the magnetic disc. In general, the base of the magnetic disc is made of synthetic resin, while the cam is usually made of a metal material. Accordingly, the expansion coefficients of both materials are different from each other.
When the coefficient of linear expansion of the magnetic disc is different from that of the cam for displacing the magnetic head, the position of the magnetic head is deviated from a desired track depending upon temperature. As a result, the reproduction output from the magnetic head is lowered.
If the reproduction output is lowered, it follows that correct information is not reproduced and there is the possibility that erroneous information is likely to be reproduced so that the reliability of the apparatus is decreased.
Furthermore, it is usual that the pitch of a track to be formed on a magnetic disc is small, and accordingly it is necessary that the track position and especially the outermost track position are detected correctly. Therefore, a highly accurate detection means is required in order to detect the track position. This means that the apparatus becomes very expensive.